Scanning particle beam instruments are, of course, well known. Such an instrument typically includes a scanner, a radiation detector and a DC amplifier, the scanner being operable to scan a beam of charged particles in a predetermined pattern relative to a specimen, the radiation detector being operable to generate a signal representative of an instantaneous intensity of radiation resulting from interaction of the beam and a specimen, and the DC amplifier being operable to amplify the signal generated by the radiation detector to produce a video signal suitable for transmission to a display device to cause an image of a scanned portion of a specimen to be displayed.
In this specification, “radiation” includes any particle or electromagnetic radiation that may be emitted by or reflected from a specimen. A “video signal” is a varying DC signal variable between a lower signal level that would be represented by a display device as black, and an upper signal level that would be represented by the display device as white. Signal levels between the lower and upper signal levels would be represented by the display device as shades of grey.
The varying DC signal that forms the video signal may be thought of as consisting of a constant DC signal component and a superimposed varying AC signal component. FIG. 1a of the accompanying drawings shows such a varying DC signal (denoted by reference numeral 4) and FIGS. 1b and 1c show, respectively, the constant DC signal component 6 and varying AC signal component 8 that make up the varying DC signal 4 of FIG. 1a. 
The radiation detector of the scanning particle beam instrument is usually subject to a DC offset, such that the signal generated by the detector includes an unwanted DC signal component. The signal generated by the detector, including the unwanted DC signal component, is amplified by the DC amplifier. The DC amplifier is itself usually subject to a DC offset, such that the video signal produced by the DC amplifier includes an unwanted DC signal component made up of the amplified DC offset introduced by the radiation detector, together with the DC offset introduced by the DC amplifier.
The DC offsets of the detector and DC amplifier are in general subject to drift so that the magnitude of the unwanted DC signal component of the video signal varies over time, the variable unwanted DC signal component of the video signal manifesting itself as regions of varying brightness in images displayed by a display device. Depending on the rate at which the DC offsets drift, and the rate at which the beam of charged particles is scanned relative to a specimen, the regions of varying brightness can be portions of individual lines of the images, entire lines or multiples of entire lines of the images, or even consecutive frames of the images, such that the images appear to flicker.
It is possible significantly to reduce the effect of the DC offset of the detector by capacitor-coupling the DC amplifier to the detector, so as to prevent the DC signal component of the signal generated by the detector, which includes the unwanted DC signal component introduced by the DC offset of the detector, from reaching the DC amplifier. This gives rise to an intermediate signal consisting of the amplified AC signal component of the signal generated by the detector and an unwanted DC signal component introduced by the DC offset of the DC amplifier.
It is then necessary to add a DC signal component to the intermediate signal to obtain the video signal. At present this is done by periodically determining a mean value of the amplified AC signal component, and adding to the amplified AC signal component a DC signal component derived from the mean value of the amplified AC signal component. This works well unless the signal generated by the detector is consistently low or consistently high during the period in which the mean value of the amplified AC signal component is determined, because this causes the mean value of the amplified AC signal component to vary, and hence the DC signal component derived from the mean value of the amplified AC signal component to vary. The variable DC signal component of the video signal also manifests itself as regions of varying brightness in images displayed by a display device.